A spectacular error can be anything. Many factors have to come together for an error to be truly spectacular. An example is a SBA dollar struck on a Sacagawea dollar brass planchet. SBA dollars were struck in 1999 and Sacagawea dollars were struck in 2000. A transitional error occurs when a coin is struck on a planchet from a previous year with different metal composition. There is also a unique Sacagawea dollar struck on a SBA dollar planchet, which is also a transitional and a spectacular error.

Images courtesy of Byers Numismatic Corp.

Significant examples:
World Famous SBA Struck on Sacagawea Brass Planchet. Transitional (illustrated above).

Sources and/or recommended reading: